Developments of chip-type or small-size electronic components are aggressively proceeding to cope with the requirement for downsizing of electronic instruments, high-density packaging of print substrates, promotion of packaging efficiency and the like. Along with the developments, requirement for production of chip-type or small-size electrolytic capacitors used as components is increasing. In this point and also in view of easy handleability, development and dissemination of solid electrolytic capacitors not using an electrolytic solution are abruptly growing in recent years.
Generally, a chip-type solid electrolytic capacitor is composed by forming an oxide dielectric film on an etched valve-acting metal foil and thereon forming cut-out grooves each in the element form (see, JP-A-5-283304 (the term “JP-A” as used herein means an “unexamined published Japanese patent application), or by a process where after fixing foils each cut out into an element shape on a metal-made support and thereon forming a solid electrolyte, a cathode electrically conducting layer comprising carbon paste and silver paste is formed thereon and an outer jacket part enclosing the whole is formed.
Among valve-acting metals such as aluminum, tantalum, niobium and titanium, aluminum is advantageous in that the surface area can be easily enlarged by an etching treatment and the oxide film formed on the surface by anodization (electrochemical formation) using the aluminum as the anode can be used as a dielectric material, therefore, a small capacitor having a large capacitance can be produced at a low cost as compared with other capacitors. By virtue of these properties, an aluminum solid electrolytic capacitor particularly for low voltage use is widely used.
Presently, the electrode foil for use in the aluminum solid electrolytic capacitor is an aluminum foil which is electrochemically or chemically etched to enlarge the surface area and then subjected to punching into the shape of a product pattern and electrochemical formation of the cut end part.
Methods for etching an aluminum foil include a DC (direct current) electrolytic etching method where an aluminum foil is etched in an electrolytic solution comprising a chloride ion-containing aqueous solution having added thereto a phosphoric acid, a sulfuric acid, a nitric acid or the like by passing a DC current using the aluminum foil as the positive electrode and an electrode disposed adjacently to the aluminum foil as the negative electrode, and an AC (alternating current) electrolytic etching method where an aluminum foil is etched in an electrolytic solution comprising a chloride ion-containing aqueous solution having added thereto a phosphoric acid, a sulfuric acid, a nitric acid or the like by passing an AC current between electrodes disposed at both sides of the aluminum foil (indirect supply of electricity) or between the aluminum foil and each of the electrodes disposed at both sides thereof (direct supply of electricity).
In the DC electrolytic etching, the etching proceeds while forming tunnel-like pits in crystallographic orientation. On the other hand, in the AC current electrolytic etching, the etching proceeds while forming etching pits sequentially connected like a rosary in random directions and this is advantageous for enlarging the surface area (area enlargement). Therefore, AC electrolytic etching is predominantly performed for the etching of an aluminum foil, however, a method of combining these two methods and a method of gradually increasing the AC voltage have been also proposed (see, JP-A-11-307400). In addition, a method involving adjustments of the waveform, amplitude and the like of the AC to improve the effective area enlargement (JP-A-7-235456) and a method where aluminum comprising a specific metal which works as a starting point of etching corrosion is used (JP-A-7-169657) have been also proposed.
After a valve-acting metal foil is formed into a porous valve-acting metal foil by electrochemical etching or after a dielectric layer is formed thereon, when the foil is cut into a capacitor element shape, cracks are generated in the porous layer formed by etching in the vicinity of cut face, and burrs are generated in the cut end part to render the part rough.
These cracks, burrs and the like on the cut edge surface generated at the time of cutting give rise to deterioration of capacitor properties.
In a step of attaching electrically conducting polymer to the foil to form a cathode part, masking is applied to the boundary between the anode-leading-out-part and the cathode part for the purpose of preventing the treating solution from creeping up to the anode-leading-out-part. However, electrically conducting polymer easily spreads beyond the masking material toward the anode part, which results in increase of leakage current.
WO 02/063645 has proposed a method where an etching layer is formed on the cut edge surface of a foil cut out into a capacitor element shape by electrolytic etching and at this time, burrs on the cut edge part are dissolved. However, in this method, etching is likely to be localized on the cut end part of the foil and this makes it difficult to control the current distribution, and another problem is involved that the cut edge part is dissolved or the strength of the part is decreased so quickly that effective area of the element decreases, failing in achieving a mass-production process of etched foils having a stable quality.